An SI injection is a shot of numbing medication and anti-inflammatory steroid delivered directly into the sacroiliac joint, one of the two joints connecting the base of your spine to the pelvis. It serves two purposes: diagnosing whether the SI joint is actually causing your lower back pain, and treating that pain by reducing inflammation inside the joint. The procedure takes only a few minutes and is done with imaging guidance to ensure the needle reaches the right spot.
Where the SI Joint Is and Why It Causes Pain
The sacroiliac joints sit on either side of your lower spine, right where the triangular bone at the base of your spine (the sacrum) meets the large pelvic bones. Each joint has a thin layer of cartilage on both surfaces and a small amount of fluid between them, similar to other movable joints in the body. Strong ligaments surround and stabilize them.
These joints absorb a tremendous amount of force during walking, running, and even sitting. When the cartilage wears down, the ligaments loosen, or the joint becomes inflamed, the result is pain that typically centers in the low back and buttock on one side. It can radiate into the hip or down the back of the thigh, which is why SI joint dysfunction is frequently mistaken for a herniated disc or hip problem.
Diagnostic vs. Therapeutic Injections
SI joint pain is notoriously difficult to pin down with imaging alone. X-rays and MRIs often look normal even when the joint is the real source of pain. That’s where a diagnostic SI injection comes in: a doctor injects a numbing agent into the joint, and if the pain drops significantly within minutes, the SI joint is confirmed as the source. If the numbing agent brings no relief, doctors look elsewhere.
A therapeutic injection uses the same approach but adds a long-acting steroid to calm inflammation inside the joint. Because a numbing agent is always included, a single injection can be both diagnostic and therapeutic at the same time. You’ll know quickly whether the joint is the problem, and if it is, the steroid begins working over the following days to provide longer-lasting relief.
What Happens During the Procedure
You lie face down on a procedure table, and the skin over the SI joint is cleaned and draped. Most injections are performed under fluoroscopic guidance, which is a real-time X-ray that lets the doctor visualize the joint as the needle advances. The SI joint has a complex, curved shape, and even experienced clinicians describe locating the joint space as the most difficult part of the procedure. Without imaging, the needle frequently misses the joint entirely.
Once the fluoroscope confirms the joint’s position, the doctor inserts a thin spinal needle into the lower portion of the joint. A tiny amount of contrast dye is injected first to verify the needle tip is actually inside the joint space. After confirmation, the steroid and numbing medication are delivered. The entire process typically takes 15 to 30 minutes, including setup.
What Recovery Looks Like
Most people feel immediate relief from the numbing agent, which is a useful sign that the joint was the pain source. That numbness wears off within a few hours, and the injection site can feel sore or even more painful than before for a day or two. The steroid component kicks in over the next several days as it reduces inflammation.
There are no major activity restrictions afterward. You can generally return to your normal routine the same day and gradually increase your activity level as pain allows. Some doctors recommend avoiding heavy lifting or intense exercise for 24 to 48 hours, but prolonged bed rest isn’t necessary.
How Well It Works and How Long Relief Lasts
When imaging guidance is used correctly, the technical success rate of getting medication into the joint is around 97%. Most people need one to three injections to see meaningful benefit. The relief itself varies widely from person to person. Clinical guidelines consider an injection successful if it provides at least 50% pain relief lasting 8 to 12 weeks.
For some people, a single injection resolves the flare and the pain doesn’t return. For others, the relief fades after a few months. If steroid injections provide only short-term improvement, doctors may suggest radiofrequency ablation, a procedure that uses heat to disrupt the nerve signals carrying pain from the joint. One study found that 50% of patients treated with radiofrequency ablation still showed improvement at 12 months, compared to steroid injection patients who did not maintain their gains at the same time point.
Platelet-rich plasma (PRP) injections are another option gaining traction. In one study, 90% of the PRP group reported being pain-free at three months, compared to 25% of the steroid group. If all injection-based treatments fail, minimally invasive fusion surgery can stabilize the joint permanently, with studies reporting pain reductions of nearly 80% at one year.
Risks and Side Effects
SI joint injections are considered low-risk. A study tracking adverse events found that the most common issue was soreness at the injection site, occurring in about 13% of patients. Pain temporarily got worse in roughly 5% of cases, and about 2% experienced facial flushing or sweating, which is a known reaction to steroid medication. A small number of patients (about 2%) had a vasovagal reaction during the procedure, meaning a brief episode of lightheadedness or near-fainting from the needle.
Rare complications include temporary weakness in the leg if the medication spreads near a nerve, and in very uncommon cases, the needle can contact the sciatic nerve during placement. These events are unusual and are further minimized by the use of fluoroscopic guidance.
How Often You Can Get Them
Medicare guidelines, which most insurers follow, allow up to two diagnostic injection sessions and up to four therapeutic injection sessions per 12-month period. Treatment with steroid injections beyond 12 months is generally not considered necessary and may trigger a medical review. If you’re needing injections that frequently, it’s typically a signal to explore longer-lasting options like radiofrequency ablation or surgical stabilization rather than continuing to rely on repeated steroid doses.